Determination of ionic self-diffusion coefficients of lithium electrolytes using the pulsed field gradient NMR

Pulsed field gradient NMR measurements were carried out to determine the self-diffusion coefficients of the Li ion (DLi) and anion (DF) by 7Li and 19F species. The DF of LiCF3SO3 in propylene carbonate (PC) solvent increased with the decrease of molar concentration and larger than DLi in whole concentration region. The DLi also increased with the concentration decrease and then saturated in the lower concentration region. The effective ionic radius of lithium estimated from the DLi was about four times larger than that of naked Li ion. This indicates the solvation effect of PC on dissociated Li ions. In order to enhance the Li ion transport number of electrolytes, two types of acidic materials, trimethoxyborane ((CH3O)3B) and trimethyleneborate ((CH2)3(BO3C3H6)2) were added into PC solution of LiCF3SO3 changing the mixing ratio of the salt to the acid. The DF decreased with the increase of (CH3O)3B content in spite of the decrease of the solution viscosity. The addition of (CH2)3(BO3C3H6)2, on the other hand, induced decrease of both DLi and DF, mainly due to the increase of the solution viscosity. These results suggest that (CH3)3BO3 is effective to increase the transport number due to the acidic trapping effect on anions.